1. Field of the Invention
The present invention relates to a battery charging apparatus, and in particular, to a battery charging apparatus for charging a battery comprising a plurality of battery cells connected to each other in series.
2. Description of Background Art
In recent years, vehicles powered by batteries have been developed. Requirements for a battery mounted in a vehicle for supplying power include small size, light weight and large capacity. In many cases, a set battery is used in order to satisfy these requirements. A set battery comprises a plurality of cell batteries which are connected to each other in series.
Such a set battery is charged by applying a voltage across the terminals of each battery cell in the series connection as will be described by referring to FIGS. 7-12. The voltage of the battery cells connected in series to each other is determined by the number of connected cells.
In general, electric charge is contained even if the battery has been fully charged, the excess energy is not accumulated in the electrodes but, instead, consumed in an electrolysis of the electrolyte which may inadvertently generate reaction gas or dissipate excessive heat. This phenomenon has an undesirable effect on the life of the set battery. In order to solve this problem, it is necessary to continuously monitor the battery during electric charging and discontinue the charging as soon as the battery achieves a fully charged state.
As shown in FIG. 7, a set battery 50 is configured to comprise a plurality of battery cells 50-1 to 50-n. A main power supply 53 is operatively connected to a variable constant-current control circuit 51 with electric power for charging the set battery 50. The variable constant-current control 51 controls the electric power supplied thereto, providing the set battery 50 with a predetermined charging current ICH.
An overvoltage detecting circuit 52 monitors the series voltage VBA of the set battery 50 in order to protect the set battery 50 against an overvoltage. As shown in FIG. 8(a), the overvoltage detecting circuit 52 supplies an overvoltage detecting signal S1 to the variable constant-current control circuit 51 when the series voltage VBA reaches an overvoltage level VTH. Upon receiving the overvoltage detection signal S1, the variable constant-current control circuit 51 either cuts off the charging current ICH as shown in FIG. 8(b), or switches to trickle charging, providing the set battery 50 with current having an amount close to the self-discharging current of the set battery 50.
FIG. 9 is a diagram showing how to charge a set battery 60 which is configured to comprise a plurality of lead battery cells 60-1 to 60-n. The same reference numerals and notations as those shown in FIG. 7 are used to denote the same components/elements or their equivalents.
In this arrangement, an intermittent control circuit 61 is employed for controlling the variable constant-current control circuit 51 by monitoring the series voltage VBA of the set battery 60. Intermittent charging is thereby repeated as denoted by reference numerals 1-4 of FIG. 10.
As shown in FIG. 10, line segment 1 denotes a state in which the variable constant-current control circuit 51 supplies a constant current until the series voltage VBA reaches a predetermined value V2. Line segment 2 is a state in which a charging current is output to sustain the series voltage VBA at the upper limit V2 thereafter. Line segment 3 denotes a subsequent state in which the charging current is reduced linearly until the series voltage VBA decreases from the upper limit V2 to a lower limit V1. Line segment 4 denotes a state in which the charging current is again increased in order to sustain the series voltage VBA at the lower limit V1.
In this intermittent charging, as the time cycle of the state 4 exceeds a predetermined period of time, the set battery is judged to have been fully charged and the electric charging is terminated accordingly.
FIG. 11 is a diagram showing how to charge a set battery 70 which is configured to comprise a plurality of Ni/Cd battery cells 70-1 to 70-n. The same reference numerals and notations as those shown in FIG. 9 are used to denote the same components/elements or their equivalents.
A peak-value detecting circuit 71 monitors the series voltage VBA of the set battery 70 in order to protect the set battery 70 against an overvoltage. As shown in FIG. 12(a), the peak-voltage detecting circuit 71 supplies a peak-value detection signal S2 to the variable constant-current control circuit 51 when the series voltage VBA reaches a peak value VP. Receiving the overvoltage detection signal S2, the variable constant-current control circuit 51 either cuts off the charging current ICH as shown in FIG. 12(b), or switches to trickle charging, providing the set battery 70 with current having an amount close to the self discharging current of the set battery 70.
As described above, the conventional charging of a set battery is accomplished by monitoring the series voltage VBA of the set battery. The electric charging is continued until the series voltage VBA reaches a level indicating that the set battery has been fully charged.
In the conventional technique described above, the series voltage VBA is used as a parameter for determining whether or not the set battery has entered a fully charged state. However, the battery cells constituting the set battery have capacities which are different from each other. Accordingly, the electric charging may be continued only because a battery cell has not been fully charged even if another battery cell has been fully charged. In such a case, the fully charged battery cell will be excessively charged, giving rise to a problem of a shortened life of the set battery.
In addition, in the conventional techniques, temperatures of the battery cells are not taken into consideration when determining the timing to start electric charging and the setting of the quantity of charging current. Accordingly, current may be charged excessively, exceeding an appropriate amount. The excessive charging current increases the amount of oxygen generated in the battery cell, lowering the electric-charging efficiency. As a result, the internal pressure increases, undesirably shortening the life of the set battery.